The present invention relates to a process for preparing nitrile elastomer surfaces for bonding by epoxy adhesives in an improved manner, and more particularly, relates to a process for treating the nitrile elastomer surfaces with a dilute solution of hydrochloric acid in a liquid vehicle for hydrolyzing the surface of the elastomer in a much milder manner than possible heretofor for providing a superior, less brittle, bond to surface.
Articles fabricated from nitrile elastomers are not usually receptive to epoxy adhesives for lamination of the elastomer or adhesion to other materials. Frequently, the method of fabrication provides a smooth surface which makes it even more difficult to promote adhesion. In the past, nitrile elastomers, for example, copolymers of acrylonitrile butadiene, or isoprene, have usually been treated with concentrated sulfuric acid to provide the surface of the elastomer with a roughness and surface energy conducive to adhesion by epoxy adhesives. However, when this is done, the surface becomes brittle and the adhesive bond that is formed is prone to hydrolysis.
The treatable surface is prepared by immersing or wetting the surface with a liquid solution which includes an acidic medium and thereafter can include removing the solution from the surface by washing with a suitable solvent. The surface is then dried and is ready to receive the epoxy adhesives.
The treating solution has three components; namely, the acidic agent, the liquid vehicle or carrier, and a wetting agent. Other components which can be employed as assistants to the treating solution include a surfactant to aid in cleansing the surface to be treated and additionally serve as a wetting agent.
The treating solution is a compound capable of rapidly hydrolyzing the surface of the elastomer at ordinary temperatures and pressures and often includes an acidic agent comprising strong acids such as mineral acids. It is particularly desirable that the acid used quickly treat the surface in a uniform manner so that the surface can be tenaciously adhered to by the epoxy adhesive.
The liquid vehicle serves as a solvent for the acidic agent and is the largest component of the treating solution and along with the organic solvent serves to wet the surface of the elastomer article being treated. A further feature of the liquid vehicle is that it must be easily evaporated so that it may be readily removed from the treated surface. Thus, the important properties of the liquid vehicle are that it should be a solvent for the acidic agent, it should be instrumental in wetting the surface of the article to be treated, and it should have an atmospheric boiling point considerably lower than temperatures harmful to the surface material.